"Sweets with less than 10g (0.35oz) of sugar" are available from MUJI. I tried "half-boiled castella". The castella looks and tastes fluffy, but it doesn't use flour.
The price is 150 yen (tax included) and the calories are 102 kcal. The sugar content is limited to 8.9g (0.32oz). We do not use any flour, but use soybean flour. By the way, when you look at the package, the alphabetical notation is "Pao De Lo", and it seems that you are conscious of the traditional Portuguese confectionery Pao De Lo, which is said to be the origin of Japanese castella.
It's so soft that it crumbles just by pushing it with your tongue, but it's hit with sweet grains here and there. The whole dough has a strong sweetness, and the egg flavor is also very strong. On the other hand, the aftertaste has a soybean-like aroma.
If you want to combine drinks, you may want to buy Japanese tea that goes well with castella, or espresso coffee at a convenience store with a little respect for the Portuguese name.